Because Scirra is not Unity. Scirra is a smaller company that earns their income from selling licenses to us. Unity is a big business company already, that doesn't rely on selling unity licenses anymore. They make money on different ways, like giving pro support, providing courses, license special tools to big studios etc. But also Unity needed many years to get this big player they are now. It always depends on your business model, too.

Probably another reason is because it technically already does work on Android/mobile through HTML5 webpage exports, so all the app export does is the (possible) performance boosts and allow you to put it on the store.